How can I go from flat to round butt?

You mentioned that you sit all day at work. One problem that causes is a posterior pelvic tilt, a postural dysfunction. Meaning that your hip bones are tilted back as if your tailbone is being tucked under you.

If you want your butt to seem bigger, you can first correct that with some exercises like dead bugs, scissor kicks, or back to wall arm slides. The major focus is keeping a neutral spine and especially maintaining a small arch in the back. Did these small things with a client with a similar problem once a week for about 5 weeks and her posture is so much better.

The previous comments and exercises suggested are spot on. But you said that you don't feel them in your butt. Again, sitting a lot can cause your butt muscles to deactivate. So when you have been squatting your butt muscles may not be turning on. A way to train them to activate is adding glute activators in your warm-ups. Mini-band walks (side to side with band around ankles), monster band walks, glute bridges, and glute pulsing are great to wake up your glutes during your warm-up. Then when you do the main exercises they will be turned on. They will begin working as they should and you will feel it.

Hack squat machine until you hear the machine hit the middle then force your way back up. This is amazing. Also plyometrics is great. Try grabbing a bench and lunging with your back foot on the bench, do this as supersets. First round 4X20 with weight then do supersets without any weight. You should feel this in your glutes. Also talk to your trainer about deadlifts. Go heavy but only with proper technique. Keep us posted. This worked for me!